The present invention relates to resist materials for use in lithography, for example, in the production of integrated circuits and particularly to polymers having pendent silicon-containing ester protecting groups. The present invention also relates to silicon-containing alcohols and esters.
It is well known in the art to produce positive photoresist formulations such as those described in U.S. Pat. Nos. 3,666,473; 4,115,128; and 4,173,470. These include alkali-soluble phenol-formaldehyde novolac resins together with light-sensitive materials, usually a substituted diazonaphthoquinone compound. The resins and sensitizers are dissolved in an organic solvent or mixture of solvents and are applied as a thin film or coating to a substrate suitable for the particular application desired.
The resin component of these photoresist formulations is soluble in aqueous alkaline solutions, but the naphthoquinone compound acts as a dissolution rate inhibitor with respect to the resin. Upon exposure of selected areas of the coated substrate to actinic radiation, however, the naphthoquinone compound undergoes a radiation induced structural transformation, and the exposed areas of the coating are rendered more soluble than the unexposed areas. This difference in solubility rates causes the exposed areas of the photoresist coating to be dissolved when the substrate is inunersed in an alkaline developing solution while the unexposed areas are largely unaffected, thus producing a positive relief pattern on the substrate.
An alternative method for forming the pattern in a resist is referred to as chemical amplification. This method is described by C. G. Willson in Introduction to Microlithography (American Chemical Society, 1994, pp. 212-231). In this method, a photoacid generator is added to a polymer containing acid-labile groups. When a coating of this mixture is exposed to actinic radiation in an imagewise fashion, the photoacid generator in those areas struck by light will produce acid, and this acid causes a reaction of the acid-labile groups in the polymer. The polymer that has reacted in this manner is rendered soluble in aqueous base, and the image can be developed in the same manner as described above. Chemically amplified resist systems typically require a much lower dose of actinic radiation to effectively develop the pattern than do the novolac/diazoquinone type resist systems.
In most instances, the exposed and developed photoresist on the substrate will be subjected to treatment by a substrate-etchant solution or gas. The photoresist coating protects the coated areas of the substrate from the etchant, and thus the etchant is only able to etch the uncoated areas of the substrate, which in the case of a positive photoresist, correspond to the areas that were exposed to actinic radiation. Thus, an etched pattern can be created on the substrate which corresponds to the pattern on the mask, stencil, template, etc., that was used to create selective exposure patterns on the coated substrate prior to development.
The relief pattern of the photoresist on the substrate produced by the method described above is useful for various applications including as an exposure mask or a pattern such as is employed in the manufacture of miniaturized integrated electronic components.
The properties of a photoresist composition which are important in commercial practice include the photospeed of the resist, development contrast, resist resolution, and resist adhesion.
Resist resolution refers to the capacity of a resist system to reproduce the smallest features from the mask to the resist image on the substrate.
In many industrial applications, particularly in the manufacture of miniaturized electronic components, a photoresist is required to provide a high degree of resolution for very small features (on the order of two microns or less).
The ability of a resist to reproduce very small dimensions, on the order of a micron or less, is extremely important in the production of large-scale integrated circuits on silicon chips and similar components. Circuit density on such a chip can only be increased, assuming photolithography techniques are utilized, by increasing the resolution capabilities of the resist.
Photoresists are generally categorized as being either positive working or negative working. In a negative working resist composition, the imagewise light-struck areas harden and form the image areas of the resist after removal of the unexposed areas with a developer. In a positive working resist composition, the exposed areas are the non-image areas. The light-struck parts are rendered soluble in aqueous alkali developers. While negative resists are the most widely used for industrial production of printed circuit boards, positive resists are capable of much finer resolution and smaller imaging geometries. Hence, positive resists are the choice for the manufacture of densely packed integrated circuits.
In the normal manner of using a positive photoresist, a single layer of this material is imaged to give a mask on the substrate, which can further be etched with a suitable etchant or used for deposition of materials, such as metals. However, due to the limitations of optical imaging systems, resolution of small patterns, on the order of two microns or less, is limited, particularly if topography is present on the substrate. It was discovered by B. J. Lin and T. H. P. Chang, J. Vac. Sci. Tech., 1979, 16, p. 1669, that this resolution can be further improved by using multilevel systems to form a portable conformable mask.
In the conventional two-layer resist system (B. J. Lin, Solid State Technol., 1983, 26 (5), p. 105), patterns with a high aspect ratio can be obtained by anistropically etching the substrate by oxygen plasma etching (O2RIE) using a thin film of the resist formed on the substrate as a mask. Hence, much importance is given to the resistance of the resist to O2RIE. Generally, those materials that form oxides upon O2RIE, typically those containing silicon (Si), are considered to have high resistance to O2RIE.
The polymers of the invention are characterized by a polymeric backbone having at least one pendent ester group having a tertiary carbon atom attached to the ester oxygen atom in which at least one substituent of the tertiary carbon atom is an organic group which comprises at least one silicon atom. The polymer compositions of the present invention are useful as resist materials for lithography, as the imaging layer, or as the top imaging layer in a bilayer resist scheme for use in the manufacture of integrated circuits. The incorporation of silicon in the polymers of the present invention enables the formation of a robust etch mask upon exposure to the oxygen plasma used in reactive ion etching processes. The silicon-containing tertiary alcohols of the present invention enable the preparation of polymers with relatively high silicon content.
In one aspect, the invention provides a polymer comprising a polymeric backbone having at least one pendent ester functional group having a tertiary carbon atom attached to the ester oxygen atom in which at least one organic substituent attached to the tertiary carbon atom comprises at least one silicon atom.
In another aspect, the invention provides a tertiary alcohol in which at least one organic substituent attached to the tertiary carbon atom comprises at least one silicon atom.
In another aspect, the invention provides an ester of a tertiary alcohol in which at least one organic substituent attached to the tertiary carbon atom comprises at least one silicon atom.
In another aspect, the invention provides a method of forming resist patterns comprising the steps of:
a) providing a polymer comprising a polymeric backbone having at least one pendent ester functional group having a tertiary carbon atom attached to the ester oxygen atom in which at least one organic substituent attached to the tertiary carbon atom comprises at least one silicon atom;
b) coating the silicon-containing polymer onto a substrate;
c) exposing the coated substrate to actinic radiation sufficient to form a latent image; and
d) developing the latent image.
The substrate may also be coated with an organic polymer base layer as a part of a bilayer resist.
The silicon-containing alcohols and esters of the invention may be converted into a variety of reactive monomers that may in turn be used to prepare polymers which can be incorporated into compositions useful as resist materials for lithography.